1. Field of the Invention
The invention relates generally to a control unit and control method for an internal combustion engine, which executes a fuel-supply cutoff control, and, more specifically to a fuel-supply cutoff control executed over an internal combustion engine of which the operating state is adjusted to control the output torque (hereinafter, this type of internal combustion engine will be referred to as “torque-demand-type internal combustion engine”).
2. Description of the Related Art
In many types of vehicles provided with an internal combustion engine (hereinafter, referred to as “engine” where appropriate), a so-called fuel-supply cutoff control, that is, a control for cutting off the fuel supply, is executed while the vehicle is decelerating in order to enhance the fuel efficiency. The fuel-supply cutoff control is executed in order to enhance the fuel efficiency by minimizing the amount of fuel that is supplied to the engine without impairing the traveling performance of the vehicle and riding comfort. Usually, when the engine speed is within a predetermined engine speed range (when the engine speed is equal to or higher than a fuel-supply cutoff speed) while the vehicle is decelerating with the engine idling, the fuel supply is cut off. More specifically, the fuel supply is cut off when a throttle valve is closed while the vehicle is traveling and the engine speed is equal to or higher than the fuel-supply cutoff speed. Also, when the engine speed decreases to a fuel-supply restart speed that is the lower limit of the engine speed range, the fuel supply is restarted in order to prevent engine stalling.
More specific description will be provided below. When the vehicle is traveling using an inertia force, that is, the vehicle is coasting, for example, when the vehicle is decelerating, the engine is forcibly rotated by an external force and kept rotating. The fuel-supply cutoff control is executed while the engine speed is in an engine speed range in which the engine keeps rotating by itself. In other words, the fuel supply is restarted when it becomes impossible for the engine to rotate by itself. Namely, while the vehicle is decelerating, the fuel supply is cut off until the engine speed decreases to the fuel-supply restart speed.
If a control is executed so that it takes longer for the engine speed to decrease to the fuel-supply restart speed, the time period in which the fuel supply is cut off is prolonged, which further enhances the fuel efficiency. Therefore, in conventional technologies, components that constitute a drive power system (power train system) from an engine to drive wheels are substantially directly connected to mechanically each other in order to minimize a decrease in the engine speed due to, so-called, slippage in the drive power system. In an example of such control, the engine speed during deceleration is made relatively high by engaging a lock-up clutch (direct-connection clutch) of a hydraulic power transmission device, for example, a torque converter.
Because the lock-up clutch connects an input-side member to an output-side member mechanically instead of using fluid, the lock-up clutch also transfers torque fluctuations without moderating them. This may cause a shock and vibration when the fuel-supply cutoff control is started.
Japanese Patent Application Publication No. JP-2001-342878 (JP-A-2001-342878) describes a control unit for an internal combustion engine, which makes it possible to minimize such shock and vibration. The control unit for an internal combustion engine executes a fuel-supply cutoff control for cutting off the fuel supply to the internal combustion engine while the vehicle is traveling. The control unit includes at least one of an immediately-before fuel-supply cutoff torque control unit and an immediately-after fuel-supply cutoff torque control unit. The immediately-before fuel-supply cutoff torque control unit decreases the torque directed from the internal combustion engine toward drive wheels in a drive power system, which is from the internal combustion engine to the drive wheels, immediately before execution of the fuel-supply cutoff control if it is determined that the fuel-supply cutoff control should be executed. The immediately-after fuel-supply cutoff torque control unit increases the torque directed from the internal combustion engine toward the drive wheels in the drive power system immediately after start of the fuel-supply cutoff control.
In the control unit for an internal combustion engine, if it is determined that the fuel-supply cutoff control for cutting off the fuel supply to the internal combustion engine when the vehicle is traveling should be executed, the torque directed from the internal combustion engine toward the drive wheels is decreased immediately before execution of the fuel-supply cutoff control. As a result, when the torque output from the internal combustion engine is decreased due to execution of the fuel-supply cutoff control, the torque directed toward the drive wheels has been decreased to a certain degree. Therefore, the amount of change in the torque received by the drive wheels, which is caused due to execution of the fuel-supply cutoff control, is reduced. When the control unit for an internal combustion engine includes the immediately-after fuel-supply cutoff torque control unit, a control for increasing the torque directed toward the drive wheels in the drive power system is executed immediately after start of the fuel-supply cutoff control. Therefore, a decrease in the torque output from the internal combustion engine and an increase in the torque caused by the immediately-after fuel-supply cutoff torque control unit cancel each other. As a result, even if the torque output from the internal combustion engine decreases due to execution of the fuel-supply cutoff control, the amount of change in the torque received by the drive wheels and the drive power system is reduced.
In a vehicle provided with an engine of which the output torque is controlled independently of an operation of an accelerator pedal performed by a driver, and an automatic transmission, a “drive power control” may be executed. In the drive power control, a target drive torque, which takes a positive value or a negative value and which is calculated based on the amount by which the accelerator pedal is operated by the driver (hereinafter, referred to as “accelerator pedal operation amount” where appropriate), the operating conditions of the vehicle, etc. is achieved by controlling the engine torque and the gear ratio of the automatic transmission. Controls such as a “drive power requiring control”, a “drive power demand control”, and a “torque-demand control” are similar to the drive power control.
A torque-demand engine control unit calculates a target torque which should be output from an engine based on the accelerator pedal operation amount, the engine speed, and the external load, and controls the fuel injection amount and the air supply amount based on the target torque. This torque-demand engine control unit actually calculates a target generation torque by adding loss load torques, such as a friction torque, that are lost in the engine and a power train system to the required output torque. The engine control unit then controls the fuel injection amount and the air supply amount so that the target generation torque is achieved. The torque-demand engine control unit improves the driving performance, for example, makes it possible to always maintain a constant driving feel, by adjusting the engine torque, which is a physical quantity that directly exerts an influence on the vehicle control, to a reference value. That is, the torque required by the entire vehicle including the engine and the power train system and the target torque are matched with each other by controlling the engine and an automatic transmission (including a lock-up clutch).
In addition, if the torque-demand control method is employed only for the engine (that is, only the engine is a control target and the automatic transmission is not a control target), only the engine is controlled to output a target torque required of the engine.
That is, the throttle valve opening amount, the ignition timing, and the fuel injection amount, at which the target torque is achieved, are calculated based on the relationship among the engine speed NE, the intake efficiency KL (=amount (mass flow) of air taken into cylinder/maximum amount (mass flow) of air that can be taken in cylinder), the ignition timing SA (hereinafter, ignition timing will be referred to as “SA” (Spark Advance) where appropriate), the air-fuel ratio A/F (stoichiometric air-fuel ratio may be used), and the torque. Namely, in the engine torque-demand control described above, an engine ECU (Electronic Control Unit) calculates a target engine torque and controls the throttle valve opening amount, the ignition timing and the fuel injection amount to achieve the target torque.
However, according to JP-A-2001-342878, only a throttle valve and an idle speed control valve are used to decrease the torque immediately before cutting off the fuel supply or increase the torque immediately after the fuel supply is restarted in response to a command to accelerate the vehicle issued by the driver. That is, the torque output from the engine is controlled only by adjusting the intake air amount. As a result, it is sometimes difficult to achieve the required torque.